"We immediately set up an internal investigation to find out what happened, and that is ongoing.

"We have involved the Metropolitan Police in that investigation."

The device was spotted in Ilbert Street, Queen's Park , and handed into the Sunday Mirror.

Among the unencrypted files found on the 2.5GB memory stick included the Queen's route when using the airport, types of ID needed to access restricted areas and the location of CCTV cameras, tunnels and escape shafts linked to the Heathrow Express.

Read More

Fears were raised over terror threats after the sensitive information was found.

A security source told The Mirror : “In the wrong hands this would represent a profound threat in terms of terrorism or espionage."

In addition, an expert told the paper: "In the wrong hands it could ­potentially be very helpful and would save them a lot of time in planning an attack.”

A former counter-terrorism chief who specialises in airport security said this was a "huge security breach" and said "there are serious questions to be answered" as to why the unencrypted memory stick was taken off site.

Keep up to date with the latest news in west London via the free getwestlondon app.

You can set up your app to see all the latest news and events from your area, plus receive push notifications for breaking news.